1. Field of the Invention
The present invention relates to an image reading device and an image reading method, in particular, a device and a method for breaking down image information on a document into minute areas and reading said image information as electrical signals for use in microfilm scanners and the like.
2. Description of the Related Art
Microfilm reader/scanners, for example, are image reading devices which read document images such as image frames photographed on microfilm. Typically, in such microfilm reader/scanners, prescanning is executed to obtain optimum reading parameters (e.g., amount of exposure, variable magnification, and the like) prior to a main scan to read a document image. The main scan starts after a predetermined time has elapsed following the completion of the prescan.
That is, first a prescan is executed to determine whether the film image is a dark image or a bright image. If the film image is a dark image, the amount of light exposure (hereinafter referred to simply as "exposure amount") is set higher during the main scan, whereas if the film image is a bright image, the exposure amount is set lower during the main scan. The film image is then read (main scan), and the main scan is started after waiting for stabilization of the set exposure amount. If there is a large difference between the exposure amount during the prescan and the exposure amount during the main scan, much time is required for the main scan exposure amount to become stabilized, i.e., the time required to attain the main scan exposure amount. Therefore, conventionally, the greatest amount of exposure change is estimated between the prescan exposure amount and the main scan exposure amount to uniformly delay the start of the main scan. In this method, however, even when the amount of change in the exposure amount is small between the prescan and the main scan such that only a short time is required to attain the exposure amount of the main scan, the start of the main scan is delayed for the same amount of time as when the difference in the exposure amount is quite large, thereby unnecessarily delaying the start of the main scan.
Japanese Patent publication No. HEI 2-15146 (i.e., Japanese Laid-Open Patent Application No. SHO 60-145763) discloses a means for measuring the rate of change of the exposure amount of a light source (exposure change measuring means) to detect whether or not the exposure quantity is stabilized, so as to start the main scan after the exposure amount is stable. This device, however, is disadvantageous inasmuch as the requirement of providing an additional exposure change measuring means increases the number of components and increases the cost of the device.